This invention relates to the field of wellhead apparatus, more particularly to wellhead housings and casing hangers which allow the wellhead housing to be installed over the casing hanger.
Prior wellhead systems are based on drilling the borehole of the well in successively smaller diameter sections and lining these sections with correspondingly smaller diameter sections of casing. Usually the wellhead housing is secured to the outermost string of casing and therefore the largest diameter casing lines the first section of the borehole of the well. The subsequent successively smaller diameter strings of casing are supported by casing hangers which sit on annular shoulders in the wellhead housing. Since the shoulders are machined in the wellhead housing, they limit the size of tools which can be passed through the wellhead housing. These dimensions and constraints do not allow the wellhead housing to be run after the casing hanger or the casing hanger to be run prior to the wellhead housing.
Prior oil and gas well completion operations in offshore locations often used a mudline wellhead positioned on the ocean floor with a few sections of casings, called a tie back string connected to a second wellhead located on a platform or similar structure. This type of completion required an adjustable sub to allow tensioning of the tie back string. The present invention eliminates the need for this adjustable sub by allowing the casing hanger to land below the landing shoulder in the wellhead housing. The tie back string is then tensioned by a tool connected to the casing hanger and the wellhead housing is then rotated to a position supporting the casing hanger.
In the event of the wellhead housing being damaged during drilling operations, previous wellhead systems prevented removal and replacement of the damaged wellhead housing due to the above-described configuration of the wellhead housing and the casing hangers. The improved wellhead system of the present invention overcomes the deficiencies of the prior art and provides a wellhead system which allows a wellhead housing to be installed over a previously installed casing hanger.
An example of a wellhead with a retractable seat is the D. L. Martin U. S. Pat. No. 3,902,743 which shows a radially movable split ring that provides a full opening through bore in its expanded position and a substantially continuous landing seat in its extended position.
The B. F. Baugh et al. U.S. Pat. No. 4,488,740 discloses a wellhead housing and a removable casing hanger support member with a plurality of circumferentially spaced breech block teeth disposed on each. The breech block teeth allow the wellhead housing to maintain a larger through bore with the casing hanger support member removed. Installation of the support member provides sufficient bearing area for the subsequent casing hangers.